Co-doped ZnO epitaxial films: from a Brillouin-like paramagnet to a phase-separated superparamagnetic ensemble.

Co-doped ZnO films are epitaxially grown on sapphire by reactive magnetron sputtering. The preparation conditions such as temperature and the composition of the sputtering gas are systematically varied. For optimized growth conditions virtually all Co dopant atoms are located on substitutional Zn lattice sites as revealed by X-ray linear dichroism (XLD). The material behaves as a Brillouin-like paramagnet with S = 3/2 and L = 1 as revealed by integral and element specific magnetometry. Reducing the oxygen content during preparation leads to the onset of phase separation as revealed by X-ray diffraction, and more clearly by a strong reduction of the XLD signal. Such samples behave like a blocked superparamagnetic ensemble. In the entire range of preparation conditions no signs of intrinsic ferromagnetism are found.

The combination of micro-resonators with spatially resolved ferromagnetic resonance.

We present two new and complementary approaches to realize spatial resolution for ferromagnetic resonance (FMR) on the 100 nm-scale. Both experimental setups utilize lithographically fabricated micro-resonators. They offer a detection sensitivity that is increased by four orders of magnitude compared with resonator-based FMR. In the first setup, the magnetic properties are thermally modulated via the thermal near-field effect generated by the thermal probe of an atomic force microscope. In combination with lock-in detection of the absorbed microwave power in the micro-resonator, a spatial resolution of less than 100 nm is achieved. The second setup is a combination of a micro-resonator with a scanning transmission x-ray microscope (STXM). Here a conventional FMR is excited by the micro-resonator while focused x-rays are used for a time-resolved snap-shot detection of the FMR excitations via the x-ray magnetic circular dichroism effect. This technique allows a lateral resolution of nominally 35 nm given by the STXM. Both experimental setups combine the advantage of low-power FMR excitation in the linear regime with high spatial resolution to study single and coupled nanomagnets. As proof-of-principle experiments, two perpendicular magnetic micro-stripes (5 µm × 1 µm) were grown and their FMR excitations were investigated using both setups.

Anti-P. acnes antibody in comedonal extracts.

Extracts of comedonal material obtained from patients with acne vulgaris were analyzed for albumin, IgG, IgM, IgA, and anti-Propionibacterium acnes antibodies using immunodiffusion and anti-immunoglobulin hemagglutination tests. Extracts were obtained from comedones of patients with (1) comedonal, (2) papulopustular, (3) nodular-cystic, (4) "burned out" acne, (5) patients under oral tetracycline treatment, and (6) with pustules. In all comedonal extracts analyzed, albumin and IgG (ratio approximately 1:2.5) could be detected. IgM, IgA, and C3 could not be found in noninflamed comedones by the techniques used; pustules, however, contained all immunoglobulin classes and albumin in ratios found in serum as well as C3. The anti-P. acnes antibody activity in the comedonal extracts was associated with the IgG class. Immunofluorescence microscopical examination of material from single comedones obtained by the cyano-acrylate technique showed IgG antibody on rod-shaped bacteria. The amount of IgG and albumin in comedonal extracts from the various patient groups was not different. It is concluded that IgG (and antibody of the IgG class) in comedones is derived from the serum and selectively accumulated in the follicle by an unknown mechanism.

Absence of intrinsic ferromagnetic interactions of isolated and paired Co dopant atoms in Zn1-xCoxO with high structural perfection.

We report element specific structural and magnetic investigations on Zn(1-x)Co(x)O epitaxial films using synchrotron radiation. Co dopants exclusively occupy Zn sites as revealed by x-ray linear dichroism having an unprecedented degree of structural perfection. Comparative magnetic field dependent measurements by x-ray magnetic circular dichroism and conventional magnetometry consistently show purely paramagnetic behavior for isolated Co dopant atoms with a magnetic moment of 4.8 (mu B). However, the total magnetization is reduced by approximately 30%, demonstrating that Co-O-Co pairs are antiferromagnetically coupled. We find no sign of intrinsic ferromagnetic interactions for isolated or paired Co dopant atoms in Co:ZnO films.